Organic electroluminescence (EL) devices utilizing organic substances are much expected to be useful as inexpensive, large-sized full color display devices of solid state emission type and many developments have been made thereon. An organic EL device is generally constructed from a light emitting layer and a pair of opposite electrodes sandwiching the light emitting layer. When an electric field is applied between the electrodes, electrons are injected from a cathode and holes are injected from an anode. The injected electrons recombine with the injected holes in the light emitting layer to form excited states. When the excited states return to the ground state, the energy is released as light.
A phosphorescent organic EL device wherein a phosphorescent organic material is used in the light emitting layer has been proposed. Utilizing the singlet excited state and the triplet excited state of the phosphorescent organic material, a high emission efficiency can be obtained by the phosphorescent organic EL device. When electrons and holes are recombined in an organic EL device, singlet excitons and triplet excitons may generate in a ratio of 1:3 in accordance with their difference in the spin multiplicity. Therefore, an organic EL device employing the phosphorescent emitting material would achieve an emission efficiency three to four times higher than that of an organic EL device employing only the fluorescent emitting material.
The early organic EL device requires a high driving voltage and is insufficient in the emission efficiency and durability. To eliminate these problems, various technical improvements have been made.
The improved emission efficiency and the prolonged lifetime are very important for reducing the power consumption of displays and improving the durability. Therefore, further improvements have been still required. In addition, many studies have been made in order to improve the emission efficiency and the device lifetime of organic EL devices employing a phosphorescent emitting material.
To solve the above problems, Patent Document 1 discloses a derivative having a 3,3′-biscarbazole skeleton as the phosphorescent host material. Patent Document 2 discloses a derivative having a 6,6′-bis(9-carbazolyl)-N,N′-disubstituted-3,3′-biscarbazole skeleton as the hole transporting material. Patent Document 3 discloses a compound having a carbazole, dibenzofuran, or dibenzothiophene skeleton as the phosphorescent host material, for example, 6,6′-bis(9-carbazolyl)-N,N′-diphenyl-3,3′-biscarbazole (Compound 32).
Patent Document 1 describes the use of the 3,3′-biscarbazole derivative in the phosphorescent light emitting layer, but describes nothing about the use in the hole transporting layer.
Patent Document 2 describes the use of the derivative having a 6,6′-bis(9-carbazolyl)-N,N′-disubstituted-3,3′-biscarbazole skeleton as the hole transporting material and a high heat stability of the derivative. However, since the derivative having such a skeleton has a large ionization potential, the driving voltage unfavorably increases if the derivative is used in the hole transporting layer adjacent to the light emitting layer.
Patent Document 3 merely describes the derivative having a carbazole, dibenzofuran, or dibenzothiophene skeleton as the phosphorescent host material, and suggests nothing about its function as the hole transporting material.
Patent Document 4 describes a compound having a biscarbazole skeleton wherein two carbazole structures are directly bonded to each other by a carbon-carbon bond (formulae (1a) and (1b)). A (hetero)arylamino group is bonded to the nitrogen atom of one carbazole skeleton through a 4,4′-biphenyldiyl group or a 9,9-dimethylfluorene-2,7-diyl group. Only in the compound 76, two aryl groups of the amino group are bonded to each other via a single bond.
Patent Document 5 describes a compound having a biscarbazole skeleton wherein two carbazole structures are directly bonded to each other by a carbon-to-carbon bond and a carbazolyl group is bonded to the nitrogen atom of the carbazole structure. The carbazolyl group bonded to the nitrogen atom must have a heterocyclic group selected from a dibenzofuranyl group, a dibenzothiophenyl group, and a carbazolyl group.